Thermoplastic polyurethanes, herein after referred to as TPUs, are well-known thermoplastic elastomers. In particular, they exhibit very high tensile and tear strength, high flexibility at low temperatures, extremely good abrasion and scratch resistance. They also have a high stability against oil, fats and many solvents, as well as stability against UV radiation and are being employed in a number of end use applications such as the automotive and the footwear industry.
As a result of the increased demand for lighter materials, a low density TPU needs to be developed which, in turn, represents a big technical challenge to provide, at minimum, equal physical properties to conventional low density PU.
It is already known to produce soles and other parts of polyurethane by a polyaddition reaction of liquid reactants resulting in an elastic solid moulded body. Up till now the reactants used were polyisocyanates and polyesters or polyethers containing OH-groups. Foaming was effected by adding a liquid of low boiling point or by means of CO2, thereby obtaining a foam at least partially comprising open cells.
Reducing the weight of the materials by foaming the TPU has not given satisfactory results up to now. Attempts to foam TPU using well-known blowing agents as azodicarbonamides (exothermic) or sodiumhydrocarbonate (endothermic) based products were not successful for mouldings with reduced densities below 800 kg/m3.
With endothermic blowing agents, a good surface finish can be obtained but the lowest density achievable is about 800 kg/m3. Also, the processing is not very consistent and results in long demoulding times. Very little or no foaming is obtained at the mould surface due to a relatively low mould temperature, resulting in a compact, rather thick skin and a coarse cell core.
By using exothermic blowing agents, a lower density foam (down to 750 kg/m3) with very fine cell structure can be achieved but the surface finish is not acceptable for most applications and demould time is even longer.
From the above it is clear that there is a continuous demand for low density TPUs having improved skin quality which can be produced with reduced demould times.
It has now been surprisingly found that foaming TPUs in the presence of thermally expandable microspheres, allows to meet the above objectives. Demould times are significantly reduced and the process can be carried out at lower temperatures, resulting in a better barrel stability. In addition, the use of microspheres even allows to further reduce the density while maintaining or improving the skin quality and demould time.
The present invention thus concerns a process for the preparation of foamed thermoplastic polyurethanes whereby the foaming of the thermoplastic polyurethane is carried out in the presence of thermally expandable microspheres and in the presence of an additional blowing agent, said microspheres containing a hydrocarbon.
The low density thermoplastic polyurethanes thus obtained (density not more than 800 kg/m3) have a fine cell structure, very good surface appearance, a relatively thin skin and show comparable physical properties to conventional PU which renders them suitable for a wide variety of applications.
The invention provides TPU products having outstanding low temperature dynamic flex properties and green strength at the time of demould, at density 800 kg/m3 and below.
The term “green strength”, as is known in the art, denotes the basic integrity and strength of the TPU at demould. The polymer skin of a moulded item, for example, a shoe sole and other moulded articles, should possess sufficient tensile strength and elongation and tear strength to survive a 90 to 180 degree bend without exhibiting surface cracks. The prior art processes often require 5 minutes minimum demould time to attain this characteristic.
In addition, the present invention therefore provides a significant improvement in minimum demould time. That is to say, a demould time of 2 to 3 minutes is achievable.
The use of microspheres in a polyurethane foam has been described in EP-A 29021 and U.S. Pat. No. 5,418,257.
Adding blowing agents during the processing of TPUs is widely known, see e.g. WO-A 94/20568, which discloses the production of foamed TPUs, in particular expandable, particulate TPUs, EP-A 516024, which describes the production of foamed sheets from TPU by mixing with a blowing agent and heat-processing in an extruder, and DE-A 4015714, which concerns glass-fibre reinforced TPUs made by injection moulding TPU mixed with a blowing agent.
Nevertheless, none of the prior art documents discloses the use of thermally expandable microspheres to improve the skin quality of foamed low density TPU (density 800 kg/m3 and even below) nor do these documents suggest the benefits associated with the present invention.